Sub displayRandomMatrix(clientsColl As Collection, resultWorkbook As Workbook)

Dim NamesRange As Range

With resultWorkbook.Worksheets("matrix_random")
...
Set NamesRange = _
    .Range(.Cells(2, 1), .Cells(clientsColl.Count + 1, 1))
Dim NamesArray() As String
ReDim NamesArray(1 To clientsColl.Count)

Dim clientRow As Long
Dim simulation As Long
clientRow = 1
simulation = 1

Dim clientCopy As client
For Each clientCopy In clientsColl
    For simulation = 1 To clientCopy.getRandomNumbers.Count
           ...
    Next

    NamesArray(clientRow) = clientCopy.getClientName
    clientRow = clientRow + 1
Next
...
NamesRange.value = NamesArray
...
End With

'debugging
Debug.Print "**************start"
For clientRow = 1 To clientsColl.Count
   Debug.Print NamesArray(clientRow)
Next
Debug.Print "**************end"

End Sub